Superalloy compositions, articles, and methods of manufacture

ABSTRACT

A composition of matter comprises, in combination, in weight percent: a largest content of nickel; at least 16.0 percent cobalt; and at least 3.0 percent tantalum. The composition may be used in power metallurgical processes to form turbine engine turbine disks.

U.S. GOVERNMENT RIGHTS

The invention was made with U.S. Government support under Agreement No.N00421-02-3-3111 awarded by the Naval Air Systems Command. The U.S.Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The invention relates to nickel-base superalloys. More particularly, theinvention relates to such superalloys used in high-temperature gasturbine engine components such as turbine disks and compressor disks.

The combustion, turbine, and exhaust sections of gas turbine engines aresubject to extreme heating as are latter portions of the compressorsection. This heating imposes substantial material constraints oncomponents of these sections. One area of particular importance involvesblade-bearing turbine disks. The disks are subject to extreme mechanicalstresses, in addition to the thermal stresses, for significant periodsof time during engine operation.

Exotic materials have been developed to address the demands of turbinedisk use. U.S. Pat. No. 6,521,175 discloses an advanced nickel-basesuperalloy for powder metallurgical manufacture of turbine disks. Thedisclosure of the '175 patent is incorporated by reference herein as ifset forth at length. The '175 patent discloses disk alloys optimized forshort-time engine cycles, with disk temperatures approachingtemperatures of about 1500° F. (816° C.). Other disk alloys aredisclosed in U.S. Pat. No. 5,104,614, US2004221927, EP1201777, andEP1195446.

Separately, other materials have been proposed to address the demands ofturbine blade use. Blades are typically cast and some blades includecomplex internal features. U.S. Pat. Nos. 3,061,426, 4,209,348,4,569,824, 4,719,080, 5,270,123, 6,355,117, and 6706241 disclose variousblade alloys.

SUMMARY OF THE INVENTION

One aspect of the invention involves a nickel-base composition of matterhaving a relatively high concentration of tantalum coexisting with arelatively high concentration of one or more other components.

In various implementations, the alloy may be used to form turbine disksvia powder metallurgical processes. The one or more other components mayinclude cobalt. The one or more other components may includecombinations of gamma prime (γ′) formers and/or eta (η) formers.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded partial view of a gas turbine engine turbine diskassembly.

FIG. 2 is a flowchart of a process for preparing a disk of the assemblyof FIG. 1.

FIG. 3 is a table of compositions of an inventive disk alloy and ofprior art alloys.

FIG. 4 is an etchant-aided optical micrograph of a disk alloy of FIG. 3.

FIG. 5 is an etchant-aided scanning electron micrograph (SEM) of thedisk alloy of FIG. 3.

FIG. 6 is a table of select measured properties of the disk alloy andprior art alloys of FIG. 3.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 shows a gas turbine engine disk assembly 20 including a disk 22and a plurality of blades 24. The disk is generally annular, extendingfrom an inboard bore or hub 26 at a central aperture to an outboard rim28. A relatively thin web 30 is radially between the bore 26 and rim 28.The periphery of the rim 28 has a circumferential array of engagementfeatures 32 (e.g., dovetail slots) for engaging complementary features34 of the blades 24. In other embodiments, the disk and blades may be aunitary structure (e.g., so-called “integrally bladed” rotors or disks).

The disk 22 is advantageously formed by a powder metallurgical forgingprocess (e.g., as is disclosed in U.S. Pat. No. 6,521,175). FIG. 2 showsan exemplary process. The elemental components of the alloy are mixed(e.g., as individual components of refined purity or alloys thereof).The mixture is melted sufficiently to eliminate component segregation.The melted mixture is atomized to form droplets of molten metal. Theatomized droplets are cooled to solidify into powder particles. Thepowder may be screened to restrict the ranges of powder particle sizesallowed. The powder is put into a container. The container of powder isconsolidated in a multi-step process involving compression and heating.The resulting consolidated powder then has essentially the full densityof the alloy without the chemical segregation typical of largercastings. A blank of the consolidated powder may be forged atappropriate temperatures and deformation constraints to provide aforging with the basic disk profile. The forging is then heat treated ina multi-step process involving high temperature heating followed by arapid cooling process or quench. Preferably, the heat treatmentincreases the characteristic gamma (γ) grain size from an exemplary 10μm or less to an exemplary 20-120 μm (with 30-60 μm being preferred).The quench for the heat treatment may also form strengtheningprecipitates (e.g., gamma prime (γ′) and eta (η) phases discussed infurther detail below) of a desired distribution of sizes and desiredvolume percentages. Subsequent heat treatments are used to modify thesedistributions to produce the requisite mechanical properties of themanufactured forging. The increased grain size is associated with goodhigh-temperature creep-resistance and decreased rate of crack growthduring the service of the manufactured forging. The heat treated forgingis then subject to machining of the final profile and the slots.

Whereas typical modern disk alloy compositions contain 0-3 weightpercent tantalum (Ta), the inventive alloys have a higher level. Thislevel of Ta is believed unique among disk alloys. More specifically,levels above 3% Ta combined with relatively high levels of other γ′formers (namely, one or a combination of aluminum (Al), titanium (Ti),niobium (Nb), tungsten (W), and hafnium (Hf)) and relatively high levelsof cobalt (Co) are believed unique. The Ta serves as a solid solutionstrengthening additive to the γ′ and to the γ. The presence of therelatively large Ta atoms reduces diffusion principally in the γ′ phasebut also in the γ. This may reduce high-temperature creep. Discussed infurther detail regarding the example below, a Ta level above 6% in theinventive alloys is also believed to aid in the formation of the η phaseand insure that these are relatively small compared with the γ grains.Thus the η precipitate may help in precipitation hardening similar tothe strengthening mechanisms obtained by the γ′ precipitate phase.

It is also worth comparing the inventive alloys to the modern bladealloys. Relatively high Ta contents are common to modern blade alloys.There may be several compositional differences between the inventivealloys and modern blade alloys. The blade alloys are typically producedby casting techniques as their high-temperature capability is enhancedby the ability to form very large polycrystalline and/or single grains(also known as single crystals). Use of such blade alloys in powdermetallurgical applications is compromised by the formation of very largegrain size and their requirements for high-temperature heat treatment.The resulting cooling rate would cause significant quench cracking andtearing (particularly for larger parts). Among other differences, thoseblade alloys have a lower cobalt (Co) concentration than the exemplaryinventive alloys. Broadly, relative to high-Ta modern blade alloys, theexemplary inventive alloys have been customized for utilization in diskmanufacture through the adjustment of several other elements, includingone or more of Al, Co, Cr, Hf, Mo, Nb, Ti, and W. Nevertheless, possibleuse of the inventive alloys for blades, vanes, and other non-diskcomponents can't be excluded.

Accordingly, the possibility exists for optimizing a high-Ta disk alloyhaving improved high temperature properties (e.g., for use attemperatures of 1200-1500° F. (649-816° C.) or greater). It is notedthat wherever both metric and English units are given the metric is aconversion from the English (e.g., an English measurement) and shouldnot be regarded as indicating a false degree of precision.

EXAMPLE

Table I of FIG. 3 below shows a specification for one exemplary alloy orgroup of alloys. The nominal composition and nominal limits were derivedbased upon sensitivities to elemental changes (e.g., derived from phasediagrams). The table also shows a measured composition of a test sample.The table also shows nominal compositions of the prior art alloys NF3and ME16 (discussed, e.g., in U.S. Pat. No. 6,521,175 and EP1195446,respectively). Except where noted, all contents are by weight andspecifically in weight percent.

The most basic η form is Ni₃ Ti. It has generally been believed that, inmodern disk and blade alloys, η forms when the Al to Ti weight ratio isless than or equal to one. In the exemplary alloy, this ratio is greaterthan one. From compositional analysis of the n phase, it appears that Tasignificantly contributes to the formation of the η phase as Ni₃ (Ti,Ta). A different correlation (reflecting more than Al and Ti) maytherefore be more appropriate. Utilizing standard partitioningcoefficients one can estimate the total mole fraction (by way of atomicpercentages) of the elements that substitute for atomic sites normallyoccupied by Al. These elements include Hf, Mo, Nb, Ta, Ti, V, W and, toa smaller extent, Cr. These elements act as solid solution strengthenersto the γ′ phase. When the γ′ phase has too many of these additionalatoms, other phases are apt to form, such as n when there is too muchTi. It is therefore instructive to address the ratio of Al to the sum ofthese other elements as a predictive assessment for n formation. Forexample, it appears that η will form when the molar ratio of Al atoms tothe sum of the other atoms that partition to the Al site in γ′ is lessthan or equal to about 0.79-0.81. This is particularly significant inconcert with the high levels of Ta. Nominally, for NF3 this ratio is0.84 and the Al to Ti weight percent ratio is 1.0. For test samples ofNF3 these were observed as 0.82 and 0.968, respectively. The η phasewould be predicted in NF3 by the conventional wisdom Al to Ti ratio buthas not been observed. ME16 has similar nominal values of 0.85 and 0.98,respectively, and also does not exhibit the η phase as would bepredicted by the Al to Ti ratio.

The η formation and quality thereof are believed particularly sensitiveto the Ti and Ta contents. If the above-identified ratio of Al to itssubstitutes is satisfied, there may be a further approximate predictorfor the formation of η. It is estimated that η will form if the Alcontent is less than or equal to about 3.5%, the Ta content is greaterthan or equal to about 6.35%, the Co content is greater than or equal toabout 16%, the Ti content is greater than or equal to about 2.25%, and,perhaps most significantly, the sum of Ti and Ta contents is greaterthan or equal to about 8.0%.

In addition to substituting for Ti as an η-former, the Ta has aparticular effect on controlling the size of the η precipitates. A ratioof Ta to Ti contents of at least about three may be effective to controlη precipitate size for advantageous mechanical properties.

FIGS. 4 and 5 show microstructure of the sample composition reflectingatomization to powder of about 74 μm (0.0029 inch) and smaller size,followed by compaction, forging, and heat treatment at 1182° C. (2160°F.) for two hours and a 0.93-1.39° C./s (56-83° C./minute (100-150°F./minute)) quench. FIG. 4 shows η precipitates 100 as appearing lightcolored within a γ matrix 102. An approximate grain size is 30 μm. FIG.5 shows the matrix 102 as including much smaller γ′ precipitates 104 ina γ matrix 106. These micrographs show a substantially uniformdistribution of the η phase. The η phase is no larger than the γ grainsize so that it may behave as a strengthening phase without thedetrimental influence on cyclic behavior that would occur if the η phasewere significantly larger.

FIG. 5 shows the uniformity of the γ′ precipitates. These precipitatesand their distribution contribute to precipitation strengthening.Control of precipitate size (coarsening) and spacing may be used tocontrol the degree and character of precipitate strengthening.Additionally, along the η interface is a highly ordered/aligned region108 of smaller γ′ precipitates. These regions 108 may provide furtherimpediments to dislocation motion. The impediment is a substantialcomponent of strengthening against time-dependent deformation, such ascreep. The uniformity of the distribution and very fine size of the γ′in the region 108 indicates this is formed well below the momentarytemperatures found during quenching.

Alloys with a high γ′ content have been generally regarded as difficultto weld. This difficulty is due to the sudden cooling from the welding(temporary melting) of the alloy. The sudden cooling in high γ′ alloyscauses large internal stresses to build up in the alloy leading tocracking.

The one particular η precipitate enlarged in FIG. 5 has an includedcarbide precipitate 120. The carbide is believed primarily a titaniumand/or tantalum carbide which is formed during the solidification of thepowder particles and is a natural by-product of the presence of carbon.The carbon, however, serves to strengthen grain boundaries and avoidbrittleness. Such carbide particles are extremely low in volumefraction, extremely stable because of their high melting points andbelieved not to substantially affect properties of the alloy.

As noted above, it is possible that additional strengthening is providedby the presence of the η phase at a size that is small enough tocontribute to precipitate phase strengthening while not large enough tobe detrimental. If the η phase were to extend across two (or more)grains, then the dislocations from deformation of both grains would bemore than additive and therefore significantly detrimental,(particularly in a cyclic environment). Exemplary η precipitates areapproximately 2-14 μm long in a field of 0.2 μm cooling γ′ and anaverage grain diameter (for the γ) of 30-45 μm. This size isapproximately the size of large γ′ precipitates as found in conventionalpowder metallurgy alloys such as IN100 and ME16. Testing to date hasindicated no detrimental results (e.g., no loss of notch ductility andrupture life).

Table II of FIG. 6 shows select mechanical properties of the exemplaryalloy and prior art alloys. All three alloys were heat treated to agrain size of nominal ASTM 6.5 (a diameter of about 37.8 μm (0.0015inch)). All data were taken from similarly processed subscale material(i.e., heat treated above the γ′ solvus to produce the same grain sizeand cooled at the same rate). The data show a most notable improvementin quench crack resistance for the inventive alloys. It is believed thatthe very fine distribution of γ′ in the region 108 around the ηprecipitate (which γ′ precipitates do not form until very lowtemperatures are reached during the quench cycle) are participating inthe improved resistance to quench cracking. A lack of this γ′ around theη might encourage the redistribution of the stresses during the quenchcycle to ultimately cause cracking.

From Table II it can be seen that, for equivalent grain sizes, thesample composition has significant improvements at 816° C. (1500° F.) intime dependent (creep and rupture) capability and yield and ultimatetensile strengths. At 732° C. (1350° F.) the sample composition hasslightly lower yield strength than NF3 but still significantly betterthan ME16. Further gains in these properties might be achieved withfurther composition and processing refinements.

A test has been devised to estimate relative resistance to quenchcracking and results at 1093° C. (2000° F.) are also given in Table II.This test accounts for an ability to withstand both the stresses andstrains (deformation) expected with a quench cycle. The test isdependent only on the grain size and the composition of the alloy and isindependent of cooling rate and any subsequent processing schedule. Thesample composition showed remarkable improvements over the two baselinecompositions at 1093° C. (2000° F.).

Alternative alloys with lower Ta contents and/or a lack of nprecipitates may still have some advantageous high temperatureproperties. For example, lower Ta contents in the 3-6% range or, morenarrowly the 4-6% range are possible. For substantially η-free alloys,the sum of Ti and Ta contents would be approximately 5-9%. Othercontents could be similar to those of the exemplary specification (thuslikely having a slightly higher Ni content). As with the higher Taalloys, such alloys may also be distinguished by high Co and highcombined Co and Cr contents. Exemplary combined Co and Cr contents areat least 26.0% for the lower Ta alloys and may be similar or broader(e.g., 20.0% or 22.0%) for the higher Ta alloys.

One or more embodiments of the present invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, the operational requirements of any particular engine willinfluence the manufacture of its components. As noted above, theprinciples may be applied to the manufacture of other components such asimpellers, shaft members (e.g., shaft hub structures), and the like.Accordingly, other embodiments are within the scope of the followingclaims.

1. A powder metallurgical gas turbine engine disk or disk substratecomprising in combination, in weight percent: a content of nickel as alargest content; at least 18.0 percent cobalt; at least 6.0 percenttantalum; and at least 0.5 percent niobium.
 2. The powder metallurgicalgas turbine engine disk or disk substrate of claim 1 wherein: saidcontent of nickel is at least 50 percent.
 3. The powder metallurgicalgas turbine engine disk or disk substrate of claim 1 wherein: saidcontent of nickel is 44-56 percent.
 4. The powder metallurgical gasturbine engine disk or disk substrate of claim 1 wherein: said contentof nickel is 48-52 percent.
 5. The powder metallurgical gas turbineengine disk or disk substrate of claim 1 further comprising: an aluminumcontent; and a titanium content, a ratio of said titanium content tosaid aluminum content being at least 0.57.
 6. The powder metallurgicalgas turbine engine disk or disk substrate of claim 1 further comprising:aluminum; titanium; and a combined content of said tantalum, aluminum,titanium, and niobium being at least 12.3 percent.
 7. The powdermetallurgical gas turbine engine disk or disk substrate of claim 1further comprising: at least 6.0 percent chromium.
 8. The powdermetallurgical gas turbine engine disk or disk substrate of claim 7further comprising: at least 2.5 percent aluminum; and no more than 4.0percent, individually, of every additional constituent, if any.
 9. Thepowder metallurgical gas turbine engine disk or disk substrate of claim7 further comprising: at least 5.8% combined of one or more of aluminum,titanium, said niobium, and hafnium.
 10. The powder metallurgical gasturbine engine disk or disk substrate of claim 7 further comprising: atleast 6.5% combined of one or more of aluminum, titanium, said niobium,and hafnium.
 11. The powder metallurgical gas turbine engine disk ordisk substrate of claim 1 further comprising: at least 2.5 percentaluminum.
 12. The powder metallurgical gas turbine engine disk or disksubstrate of claim 11 further comprising: at least 1.5 percent titanium.13. The powder metallurgical gas turbine engine disk or disk substrateof claim 1 further comprising: at least 1.5 percent titanium.
 14. Thepowder metallurgical gas turbine engine disk or disk substrate of claim1 further comprising: at least 1.5 percent tungsten.
 15. (canceled) 16.(canceled)
 17. A process for forming the powder metallurgical gasturbine engine disk or disk substrate of claim 1 comprising: compactinga powder having the composition of claim 1; forging a precursor formedfrom the compacted powder; and machining the forged precursor.
 18. Theprocess of claim 17 further comprising: heat treating the precursor, atleast one of before and after the machining, by heating to a temperatureof no more than 1232° C. (2250° F.)
 19. The process of claim 17 furthercomprising: heat treating the precursor, at least one of before andafter the machining, the heat treating effective to increase acharacteristic γ grain size from a first value of about 10 μm or less toa second value of 20-120 μm.
 20. (canceled)
 21. A powder metallurgicalgas turbine engine disk or disk substrate comprising in combination, inweight percent: a largest content of nickel; at least 16.0 percentcobalt; at least 20.0 percent combined cobalt and chromium; and at least6.0 percent tantalum.
 22. The powder metallurgical gas turbine enginedisk or disk substrate of claim 21 further comprising: at least 5.8%combined of one or more of aluminum, titanium, niobium, and hafnium. 23.A process for forming powder metallurgical gas turbine engine disk ordisk substrate comprising: compacting a powder having the composition ofclaim 21; forging a precursor formed from the compacted powder; andmachining the forged precursor.
 24. A powder metallurgical gas turbineengine disk or disk substrate comprising in combination, in weightpercent: from about 18.0 percent to about 21.0 percent cobalt, fromabout 8.5 percent to about 11.0 percent chromium, from about 6.5 percentto about 8.5 percent tantalum, from about 2.2 percent to about 2.75percent tungsten, from about 2.5 percent to about 3.4 percentmolybdenum, from about 0.03 percent to about 0.07 percent zirconium,from about 0.8 percent to about 2.0 percent niobium, from about 2.0percent to about 2.75 percent titanium, from about 3.0 percent to about3.5 percent aluminum, from about 0.02 percent to about 0.07 percentcarbon, from about 0.02 percent to about 0.06 percent boron; and balancenickel and minor amounts of impurities.
 25. (canceled)
 26. (canceled)27. A powder metallurgical gas turbine engine disk or disk substratecomprising in combination, in weight percent: a content of nickel as alargest content; at least 16.0 percent cobalt; at least 26.0 percentcombined cobalt and chromium; at least 3.0 percent tantalum; at least0.5 percent niobium; an aluminum content; and a titanium content, aratio of said titanium content to said aluminum content being at least0.57.
 28. (canceled)
 29. The gas turbine engine disk or disk substrateof claim 27 being one of: an integrally-bladed disk wherein blades areunitarily formed with a disk body; and a disk having a circumferentialarray of blade attachment features.
 30. The powder metallurgical gasturbine engine disk or disk substrate of claim 21 further comprising, byweight: 3.0-3.5 percent aluminum; 8.5-11.0 percent said chromium;2.5-3.4 percent molybdenum; and 0.8-2.0 percent niobium.
 31. The powdermetallurgical gas turbine engine disk or disk substrate of claim 30further comprising, by weight: 2.0-2.75 percent titanium; 2.2-2.75percent tungsten; and 0.03-0.07 percent zirconium.
 32. The powdermetallurgical gas turbine engine disk or disk substrate of claim 30further comprising, by weight up to: 0.005 percent copper; 0.1 percentiron; 0.5 percent hafnium; 0.005 percent manganese; and 0.1 percentsilica.
 33. A powder metallurgical gas turbine engine disk or disksubstrate comprising in combination, in weight percent: from 18.0percent to 21.0 percent cobalt, from 8.5 percent to 11.0 percentchromium, from 6.5 percent to 8.5 percent tantalum, from 2.2 percent to2.75 percent tungsten, from 2.5 percent to 3.4 percent molybdenum, from0.03 percent to 0.07 percent zirconium, from 0.8 percent to 2.0 percentniobium, from 2.0 percent to 2.75 percent titanium, from 3.0 percent to3.5 percent aluminum, from 0.02 percent to 0.07 percent carbon, from0.02 percent to 0.06 percent boron; and nickel as a largest content. 34.(canceled)
 35. (canceled)
 36. (canceled)